January 8, 1999
Johnson Space Center, Houston, TX
Bacteria grown in chambers that simulated natural growth conditions deep beneath the Earth's surface show remarkable similarities to features seen as signs of possible primitive life in a 4.5-billion-year-old Martian meteorite, according to a paper published by a NASA/Department of Energy/university research team.
Some of the bacteria taken from deep underground in Washington state not only grew and reproduced in the laboratory chambers, but also died and became fossilized in only 8 weeks. Some of these terrestrial bacteria, as well as parts of bacteria called filaments, were mineralized (turned into fossils) and were very similar to some of the features previously interpreted as possible fossilized Martian organisms.
"Some of the fossilized forms [grown in the laboratory experiments] are strikingly similar in size and shape to features we observed in 1996 in Martian meteorite ALH84001," said Kathie Thomas-Keprta. Keprta, of Lockheed Martin Space Mission Systems and Services in Houston, is lead author of the article in the journal Geology. When a Johnson Space Center/Stanford University team reported two years ago that features in the meteorite might indicate ancient primitive life on Mars, the small sizes and shapes of the possible fossilized features drew considerable criticism because none had been reported previously from geologic sites on Earth.
"The average sizes of the microorganisms found in the Washington state rocks are well within the range of sizes seen in terrestrial bacteria," said Dr. Everett Gibson, a NASA planetary scientist and member of the original team of investigators. The longest shapes identified as Martian fossils are approximately 1/25 the diameter of a human hair, similar in size to the Washington state bacteria.
The authors contend that cell remnants in the 0.1 to 0.2 micron size range, which were produced when cells died and decayed, can be fossilized within rocky pools of water beneath the Earth's surface. A micron is 1-millionth of a meter. Some of these fossilized organisms appear to be hollow, because minerals were deposited on the outer-most walls of the organisms. When the cell walls degrade they no longer are detectable, leaving a hollow structure consisting of minerals.
The observation of mineralized fossils in underground pools on Earth may be relevant to Mars because early in the red planet's history there appears to have been abundant water on and beneath the surface. The 4.2-pound, potato-sized ALH84001 meteorite was dated to about 4.5 billion years, the period when Mars formed. The meteorite is believed to have originated beneath the Martian surface and to have been extensively fractured by impacts as meteorites bombarded the planets in the early inner solar system. Between 3.6 billion and 4 billion years ago, a time when it is generally thought the planet was warmer and wetter, water is believed to have penetrated fractures in the subsurface rock, possibly forming an underground water system.
This underground Martian water system may have been the best location for life during much of Mars history. The Columbia River basalt of Washington state is an Earth example of life thriving at depth in fractured rock where it is isolated from light and protected from any harsh environment at the surface. As such, it may be a good example of possible life on Mars.
The paper also reports that about 30 percent of the cells grown in the laboratory had attached appendages. In addition, there were filaments not attached to cells - most likely bacterial appendages - that were mineralized. The chemical composition of these unattached mineralized appendages -- much smaller than normal bacteria -- is nearly identical to the mineralized organisms. Johnson Space Center planetary scientist Dr. David McKay said this is the first time such tiny parts of bacteria are shown to become mineralized in a way identical to whole bacteria cells. "These mineralized parts may be found in the Earth's fossil record in the rocks, and should be searched for along with whole bacteria in ancient rocks," McKay said.
Identifying biological features altered by the presence of minerals is important in determining whether specific features in terrestrial rocks and in meteorites may have a biological origin. "We believe we have clearly shown that features similar in size and morphology to those in the Martian meteorite do exist within the biological record on Earth," Thomas-Keprta said, although she said this does not prove definitively that the features in the meteorite are biological in origin.
The researchers examined four types of bacteria grown on basalt rocks extracted from 1,640 feet (0.5 kilometers) within the Columbia River region in Washington state. Basalt is a type of volcanic rock typically found in lava flows, which means they formed at high temperatures too hot for bacteria to live on when the rocks formed. After extracting the bacteria-containing ground water and small rock chips, scientists cultured and grew bacteria on the rocks in the laboratory for two months or more. Conditions in the growth chambers simulated natural growth conditions far below the Columbia River area, which is a possible terrestrial model for a Martian subsurface ecosystem.
"These experiments show that mineralization of biological material can occur in as few as eight weeks, parts or pieces of bacteria can become mineralized, cell walls are not necessarily preserved when bacteria become mineralized, and features too small to be bacteria may be parts of bacteria found in both the terrestrial and Martian rock record," Thomas-Keprta said. The range of sizes and shapes of the biological filaments on the Columbia River Basalt rocks overlaps with that found on the ALH84001 meteorite. "The findings support our belief that the Martian features may be mineralized remains of extremely tiny cells, or the mineralized filaments and appendages are from larger cells."
Other scientists included Susan Wentworth and Dr. Carlton Allen, Lockheed Martin Space Mission Systems and Services in Houston; Dr. Todd Stevens, Battelle Pacific Northwest Laboratory, Richland, Wash.; Anne Taunton, University of Wisconsin Department of Geology, Madison; Dr. Annette Coleman, Brown University Biomedical Department, Providence, R.I.; and Dr. Christopher Romanek, Savannah River Ecology Laboratory, University of Georgia, Aiken, S.C.
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